Advent of Code 2020

Day 1 - Report Repair

Part 1 - Find two numbers in your list which sum to 2020

• Store the list into a dictionary.
• Look at each number in turn and subtract it from 2020.
• Check to see if that number in the list.

Notes

• One number must be more than 1010 and the other less than 1010

Part 2 - Find three numbers in your list which sum to 2020

• Same idea as part 1 but with an extra loop
• Number of checks required can be reduced by sorting the list

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Day 2 - Password Philosophy

Parts 1 and 2 - How many passwords are valid according to their policies?

• Used regular expressions to parse the input.

• Different policy is required for part 2. Reused code by passing in the desired policy as a function

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Day 3 - Toboggan Trajectory

Part 1 - How many trees would you hit moving right 3, down 1?

• Forest wraps around (right->left->right) so the toboggan's x positions needs to be mod the forest with.

Part 2 - How many trees would you hit with multiple slopes?

• Same code as part 1, but passing in different slopes (other than 3,1)

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Day 4 - Passport Processing

Part 1 - Number of passports which contain all fields

• Nice trick when parsing the input which is to separate the passports with .split('\n\n')

Part 2 - Number of valid passports

• Created a function called is_in_range and then used partial from functools in order to supply the first argument.

• Used regular expressions for other validations

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Day 5 - Binary Boarding

Part 1 - What is the highest seat ID on a boarding pass?

• Trick here is to spot the seat ids are just binary numbers!

Part 2 - What is the ID of your seat?

• Found it easier to ignore the seats with IDs +1 and -1 from yours will be in your list. in the problem statement!

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Day 6 - Custom Customs

• The idea is to use the intersection of sets to give you anyone who answered yes to a question

• Part 2 is the same, but using the union of sets

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Day 7 - Handy Haversacks

• As the definitions of which bags contain which other bags are nested, it means that recursion should be used.

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Day 8 - Handheld Halting

• Very simple VM with just three instructions and two registers (accumulator and instruction pointer)

• Part 2 is harder as one instruction is corrupted. I just brute forced changing the instructions one by one.

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Day 9 - Encoding Error

• Requires a sliding window of length 25 over the list of numbers